On Thu, Apr 29, 2021 at 05:06:41PM +0100, Steven Price wrote: > On 28/04/2021 18:07, Catalin Marinas wrote: > > I probably asked already but is the only way to map a standard RAM page > > (not device) in stage 2 via the fault handler? One case I had in mind > > was something like get_user_pages() but it looks like that one doesn't > > call set_pte_at_notify(). There are a few other places where > > set_pte_at_notify() is called and these may happen before we got a > > chance to fault on stage 2, effectively populating the entry (IIUC). If > > that's an issue, we could move the above loop and check closer to the > > actual pte setting like kvm_pgtable_stage2_map(). > > The only call sites of kvm_pgtable_stage2_map() are in mmu.c: > > * kvm_phys_addr_ioremap() - maps as device in stage 2 > > * user_mem_abort() - handled above > > * kvm_set_spte_handler() - ultimately called from the .change_pte() > callback of the MMU notifier > > So the last one is potentially a problem. It's called via the MMU notifiers > in the case of set_pte_at_notify(). The users of that are: > > * uprobe_write_opcode(): Allocates a new page and performs a > copy_highpage() to copy the data to the new page (which with MTE includes > the tags and will copy across the PG_mte_tagged flag). > > * write_protect_page() (KSM): Changes the permissions on the PTE but it's > still the same page, so nothing to do regarding MTE. My concern here is that the VMM had a stage 1 pte but we haven't yet faulted in at stage 2 via user_mem_abort(), so we don't have any stage 2 pte set. write_protect_page() comes in and sets the new stage 2 pte via the callback. I couldn't find any check in kvm_pgtable_stage2_map() for the old pte, so it will set the new stage 2 pte regardless. A subsequent guest read would no longer fault at stage 2. > * replace_page() (KSM): If the page has MTE tags then the MTE version of > memcmp_pages() will return false, so the only caller > (try_to_merge_one_page()) will never call this on a page with tags. > > * wp_page_copy(): This one is more interesting - if we go down the > cow_user_page() path with an old page then everything is safe (tags are > copied over). The is_zero_pfn() case worries me a bit - a new page is > allocated, but I can't instantly see anything to zero out the tags (and set > PG_mte_tagged). True, I think tag zeroing happens only if we map it as PROT_MTE in the VMM. > * migrate_vma_insert_page(): I think migration should be safe as the tags > should be copied. > > So wp_page_copy() looks suspicious. > > kvm_pgtable_stage2_map() looks like it could be a good place for the checks, > it looks like it should work and is probably a more obvious place for the > checks. That would be my preference. It also matches the stage 1 set_pte_at(). > > While the set_pte_at() race on the page flags is somewhat clearer, we > > may still have a race here with the VMM's set_pte_at() if the page is > > mapped as tagged. KVM has its own mmu_lock but it wouldn't be held when > > handling the VMM page tables (well, not always, see below). > > > > gfn_to_pfn_prot() ends up calling get_user_pages*(). At least the slow > > path (hva_to_pfn_slow()) ends up with FOLL_TOUCH in gup and the VMM pte > > would be set, tags cleared (if PROT_MTE) before the stage 2 pte. I'm not > > sure whether get_user_page_fast_only() does the same. > > > > The race with an mprotect(PROT_MTE) in the VMM is fine I think as the > > KVM mmu notifier is invoked before set_pte_at() and racing with another > > user_mem_abort() is serialised by the KVM mmu_lock. The subsequent > > set_pte_at() would see the PG_mte_tagged set either by the current CPU > > or by the one it was racing with. > > Given the changes to set_pte_at() which means that tags are restored from > swap even if !PROT_MTE, the only race I can see remaining is the creation of > new PROT_MTE mappings. As you mention an attempt to change mappings in the > VMM memory space should involve a mmu notifier call which I think serialises > this. So the remaining issue is doing this in a separate address space. > > So I guess the potential problem is: > > * allocate memory MAP_SHARED but !PROT_MTE > * fork() > * VM causes a fault in parent address space > * child does a mprotect(PROT_MTE) > > With the last two potentially racing. Sadly I can't see a good way of > handling that. Ah, the mmap lock doesn't help as they are different processes (mprotect() acquires it as a writer). I wonder whether this is racy even in the absence of KVM. If both parent and child do an mprotect(PROT_MTE), one of them may be reading stale tags for a brief period. Maybe we should revisit whether shared MTE pages are of any use, though it's an ABI change (not bad if no-one is relying on this). However... Thinking about this, we have a similar problem with the PG_dcache_clean and two processes doing mprotect(PROT_EXEC). One of them could see the flag set and skip the I-cache maintenance while the other executes stale instructions. change_pte_range() could acquire the page lock if the page is VM_SHARED (my preferred core mm fix). It doesn't immediately solve the MTE/KVM case but we could at least take the page lock via user_mem_abort(). Or maybe we just document this behaviour as racy both for PROT_EXEC and PROT_MTE mappings and be done with this. The minor issue with PROT_MTE is the potential leaking of tags (it's harder to leak information through the I-cache). -- Catalin _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
